1. Field of the Invention
The present invention relates to a rubber sheet joining apparatus and method for joining the end surfaces of one or two rubber sheets with each other by butt joint. In particular, it relates to a rubber sheet joining apparatus and method capable of manufacturing a jointed plane rubber sheet or a round or endless rubber band easily and efficiently without making any bulge at the jointed portion thereof.
2. Discussion of the Related Art
Generally, for vehicular tires, inner liners which are composite rubber sheets made from rubber only and body or carcass plies, belts and the like which are rubber sheets reinforced by fibers or steel cords are often used by jointing end surfaces of rubber sheet strips. For example, the rubber sheet for a body ply, a belt or the like having a predetermined length can be manufactured by making by a calendar rolling machine a roll of a long rubber textile or cloth having numerous cords embedded in the rubber layer, then by uncoiling and cutting the roll of the long rubber cloth which has the cords arranged parallel in the longitudinal direction thereof, to those sheet strips of the length corresponding to the width of the body ply or the belt which width depends on the kind and dimension of the tire to be manufactured, and further, by mutually jointing the end edges parallel to the cords of those rubber sheet stripes so cut.
Used generally for jointing end edges of two adjoining sheets are two methods called “lap joint” and “butt joint”. The lap joint can be performed by lapping an end edge of a second sheet on an end edge of a first sheet and then, by applying a pressure on a jointed area including the lapped area.
In the lap joint method, since the thickness of the rubber sheet at the lapped joint area becomes as twice the thickness of other areas, doubled are not only the thickness of the rubber sheet, but also the number of the reinforcing cords in the case of a fiber or steel cord reinforced rubber sheet. Thus, the thickness of the jointed rubber sheet dose not become uniform and the difference in rigidity becomes large, so that the uniformity of the tire is degraded. For this reason, the use of the lap joint method tends to decrease these days.
On the other hand, the butt joint can be performed by thrusting the end edge of one sheet against the end edge of the other sheet and then, by pressuring them to be firmly jointed by mechanical or physical means. Thus, a bulge is generated at the jointed portion.
That is, as described in Japanese Unexamined Published Patent Application No. 2004-142219, since the both end edges of first and second sheets are jointed as being drawn to come closer to each other by a pair of rollers each taking a bevel wheel shape, the bulge is generated on the jointed portion.
In order to level out the bulge on the jointed portion, the technology described in the foregoing Japanese application is designed to heighten the flatness at the jointed portion by adding as a step subsequent to the sheet jointing a leveling or smoothening step of pressing down the jointed portion by upper and lower leveling plates including warming means with the jointed portion being put therebetween from upper and lower sides.
However, in the technology described in the foregoing Japanese application, the flatness on the jointed portion is obtained by chemically changing the quality of the rubber by the use of the upper and lower leveling plates including the warming means. Thus, the jointed portion and other portions of the sheet become the same in thickness, but different in quality. Therefore, where tires are manufactured using sheets each jointed by the foregoing jointing method, it results that a portion which is not uniform in physical property with other portions is present on the circumference of the tire, and this causes an obstruction in enhancing the tire quality. In addition, the addition of the smoothening step causes not only an increase in the cost of the manufacturing facilities, but also an elongation in the manufacturing cycle time.
Further, as described in Japanese Unexamined Published Patent Application No. 2002-11805, there has also been known a jointing apparatus and method for butt-jointing the circumferential opposite end portions of an unvulcanized rubber sheet blank such as carcass or body ply used as tire component blank or the like on a shaping drum. In the apparatus and method described in the Japanese application, a belt-like rubber blank cut to a predetermined length is wound on the circumferential surface of the shaping drum to cling thereto with jointing portions of the shaping drum held opened, and joint end portions of the belt-like rubber blank are drawn toward each other to be temporarily jointed by closing or diametrically contracting the shaping drum, in which state the joint end portions are pressure-jointed by a pair of rollers each taking a bevel wheel shape.
In the last mentioned apparatus and method, since the joint end portions of the belt-like shape rubber blank are jointed as they are made by the rollers of the bevel wheel shape to come close to each other, it necessarily results that the jointed portion has a bulge thereon and is degraded in flatness. Accordingly, in order to level out the bulge on the jointed portion, it becomes necessary to heighten the flatness at the jointed portion by adding as a step subsequent to the sheet jointing a smoothening step of pressing down the jointed portion by leveling plates including warming means with the jointed portion being put therebetween from upper and lower sides, as described in the first mentioned Japanese application.
As described above, in the last mentioned apparatus and method, in order to level out the bulge on the jointed portion, it is required to improve the flatness by chemically degenerating the rubber by the use of the leveling plates including warming means, as described in the first mentioned Japanese application. Accordingly, where tires are manufactured using carcass or body plies each jointed by the last mentioned jointing apparatus and method, each tire would have at the circumferential portion thereof a portion which lacks uniformity because of being different in physical property, thereby making an obstruction in improving the tire quality. In addition, the addition of the smoothening step undesirably causes not only an increase in the cost for tire manufacturing facilities but also an elongation in the manufacturing cycle time.